The effect of surface tension on the growth of Lactobacillus bulgaricus and Lactobacillus acidophilus.

The close similarity, both morphologically and culturally, between Lactobacillus bulgaricus and Lactobacillus acidophilus, makes differentiation between these two important members of the aciduric group of bacteria difficult. Their similarity is so close as to have led some investigators, to conclude from earlier work that they were identical. A differentiation based on the fermentation of maltose, and maltose, saccharose and levulose, is admittedly more or less variable. Unpublished work done in these laboratories on sugar fermentations of these organisms likewise gave variable results. It would seem, then, that a differentiation of these two types of organisms based on sugar fermentations is not absolutely infallible, and a method of distinguishing between these two closely related organisms is most desirable. A review of the voluminous literature on the implantation of aciduric bacteria in the intestinal tract seems to justify the conclusion that Lactobacillus bulgaricus cannot be implanted in the intestinal tract, while implantation of Lactobacillus acidophilus is more or less easily accomplished. That the bile salts may play some part in this natural selection suggested itself, and as they are surface depressants, separation on this basis was attempted. Fifteen strains of Lactobacillus bulgaricus and fifteen strains of Lactobacillus acidophilus gathered from authentic sources were employed in this work. Sodium ricinolate and sodium taurocholate were used as surface tension depressants in a medium favorable for growth of all strains. All of our strains of Lactobacillus acidophilus grew very well in a medium of a surface tension as low as 36 dynes, while Lactobacillus bulgaricus in the same medium depressed to a surface tension of forty dynes, failed to show growth after seven days'incubation at 37° C. A surface tension of forty dynes represents the extreme lower limit for Lactobacillus bulgaricus as most of our strains were inhibited above this value.